(1) Field of the Invention
This invention relates to an ink jet printer for recording patterns of letters, figures, etc., with ink droplets and an ink droplet projecting device used in this printer.
(2) Description of the Prior Art
An ink jet printer, being of non-impact type, has excellent outstanding features including producing low noise, permitting use of common papers as the recording media, and enabling easy realization of color prints and so forth. Such printers presently available for practical use include electric charge control type, electric field control type, on-demand type and ink mist type. This invention relates to the on-demand type ink jet printer which is advantageous in that ink unused for printing need not be withdrawn, that colour printer can be very easily made and that shade of color may be varied by altering the diameter of the ink droplet.
First, the basic constructions of the conventional ink heads are described.
As the conventional ink heads, the single cavity type (U.S. Pat. No. 3,946,398) having an ink cell 91 formed of one compartment which is filled with ink as shown in FIG. 20 and the double cavity type (U.S. Pat. No. 3,747,120) having an ink cell 91 formed of two compartments as shown in FIG. 21 are famous.
The operating principle common to both these types lies in that a piezo-electric element 92 is sinuously vibrated by giving electric signals to the piezo-electric element 92; by this vibration, the pressure inside the ink cell 91 is increased, so that an ink column 94 is projected through an orifice 93 formed in part of the ink cell 91; then, the ink droplet 95 formed by separating from the ink column 94 on account of its own energy and surface tension is let fly toward the recording medium (not shown) which is placed opposite the orifice 93.
In the single cavity type, the projecting velocity of the ink droplet 95 is as low as 2-3 KHz. For this reason, when making the recording at high speeds, use of a plurality of ink heads is required, and scaling-up, increase in the number of parts and fall in reliability are inevitable. Moreover, there are such difficulties that, for example, some contrivance may be required to prevent the weakening of the injection power resulting from inverse transmission of the pressure wave to the ink feeding system 96 for replenishing ink into the ink cell 91, when the piezo-electric element 92 is curved to intrude into the ink cell 91, thereby projecting the ink droplet 95.
On the contrary, in the double cavity type which has the effect of overcoming the difficulty in said single cavity type, a partition plate 98 for separating the ink cell 91 into the compartment 91a on the piezo-electric element 92 side and the external compartment 91b on the orifice 93 side is necessary, and the coupling aperture 97 bored at the center of this partition plate 98 to communicate these two compartment 91a and 91b with each other and the orifice 93 and the coupling aperture 97 should be aligned. The minimum resolution for ordinary recording media is said to be, for example, 6 lines/mm in X-Y plotter, etc; in order to meet this condition, the bore of the orifice 93 needs to be of the order of 50 .mu.m, and the inside diameter of the coupling aperture 97 should be on the same order. Thus two holes of the size of the order of 50 .mu.m need to be aligned. This inevitably requires precision work done by use of a microscope. If the alignment between these holes is inaccurate, the desired ink droplet projecting can not be achieved.
Furthermore, the conventional ink heads have a following problem. After the projection of an ink droplet, sometimes air is sucked in through the orifice 93, thereby forming air bubbles because the piezo-electric element 92 curves in the opposite direction to that at the time of projection. Due to the high frequency vibration of the piezo-electric element 92, air dissolved in ink is separated, forming air bubbles. Since such air bubbles absorb the pressure waves produced by the vibration of the piezo-electric element 92, an undesirable phenomenon in which ink droplets are not projected in spite of the vibrations of the piezo-electric element 92 sometimes takes place. The formation of air bubbles resulting from the former reason is particularly prominent in the single cavity type. This drawback is countered by devising such means as orienting to one direction of the fluid resistance of the ink feeding system or increasing the fluid resistance in the counterflow direction of ink. The formation of air bubbles stemming from the latter reason is dealt with by preliminarily reducing the dissolved air by way of adding oxygen absorber to ink or subjecting ink to a defoaming treatment, as disclosed in Japanese Patent Publication No. 20882 of Showa 53 (1978). Whichever method is ineffective to get rid of air absorbed by the ink cell due to an impact force independent of the recording operation applied on the ink head for some reason.